I think you should rather use 25-30% in your calculations. ASICS are coming fast now, and will skyrocket the difficulty.
Same hashrate comes up: 11m, Hashrate 11/th/s with a 1th/s increase. After 2016 blocks, difficulty is increased by 9,09%.
If you suggest a cumulativ increase of additional percentages, you need to think about the costs. 360 million is RIDICULOUS. I have written about this before.
You would have to add the total hashrate of June to accomodate October increases. Just think about that. Then you would do it again, and again. Adding the current total network hashrate several times over, with increasing the increase absolute number every time.
I see what you mean, but I'm thinking that difficulty is going into an exponential curve right now. It will not last for a long time, and there will be a new plateau. What that plateau will be, we will see some time late summer/fall. After that, I think difficulty-increases might become lower than the "regular" 10-15%
Remember that those who are switching from CPU's to ASIC are likely to increase their hashing-power by a factor of 10.